1. Field of the Invention
This invention relates to light measuring devices having display elements on the path of light to be measured, and more particularly to a device for correcting the measured value of light depending on activation or de-activation of the display element.
2. Description of the Prior Art
Known cameras have been provided with finders having display marks indicating the position of an object image relative to the focal plane within the finder's field of view. Such marks generally do not overlap the object image in the field of view of the finder. However, since the marks indicate the direction which a camera distance adjusting ring must be turned, as well as when an in-focus condition is reached, the location of the marks out of the frame of the viewfield of the finder is inconvenient when the photographer focuses the camera visually.
Recently, a so-called superimposed display device has been proposed. The superimposed display superimposes the marks on the object image. To enable the photographer, while looking at the image in the range finder at the center area of the viewfield of the finder, to adjust the distance adjusting ring on the basis of the output of the range finder, this device makes use of a liquid crystal light shutter display plate arranged adjacent to the focusing screen. This liquid crystal plate displays either of two marks in the neighborhood of the range finder area indicating the respective needed direction of rotation of the distance adjusting ring, i.e., clockwise or counterclockwise. When the in-focus condition is detected, the two marks are displayed simultaneously.
However subsequent continuous presentation of these display marks in superimposed relation to the object image in the field of view of the finder may produce eyestrain when shooting. To overcome this drawback, the inventors of the present invention have developed a display device, disclosed in U.S. patent application Ser. No. 613,592, which renders the superimposed display marks entirely transparent within a prescribed time after the in-focus condition is reached. The display of the in-focus condition is then continued by another display element located outside the field of view of the finder. By using this device, quick and easy visual focusing is facilitated by a focusing direction indicating mark in superimposed form. When the in-focus condition is established and detected it is first indicated by the superimposed display to inform the photographer positively that the in-focus state has been reached. Subsequently, the "in-focus" display is transferred from the superimposed to another display element. Thus, obstacles in shooting are overcome.
In a light measuring device where light measurement is performed behind the superimposed display means, for example, in a single lens reflex camera, when the superimposed display is switched off and the external display element is switched on, the illumination on the light receiving element is changed. This creates the problem of suddenly changing the output of the light measuring circuit.